Sintered alumina is commonly employed in the production of refractory shapes, spark plugs and electrical insulating elements. Most of these applications require an alumina product of low porosity and high purity. These aluminas are usually prepared from alumina hydrate derived from the Bayer process, which involves the digestion of aluminiferous ores with a caustic solution, such as sodium hydroxide. Digestion results in a sodium aluminate solution from which the alumina hydrate is recovered by precipitation. The resulting alumina hydrate is usually contaminated with sodium impurities and the impurity content, calculated as Na.sub.2 O, is generally in the range of about 0.2-1.0% by weight on Al.sub. 2 O.sub.30 basis. This impurity content is undesirable for many applications, particularly when the sintered alumina made from this type of hydrate is employed for refractory or electrical end uses. It has already been known that when a boron compound, such as B.sub.2 O.sub.3, is incorporated in alumina hydrate and the mixture is calcined, at high temperatures a significant quantity of the sodium impurity is removed in volatile sodium borate form. Thus, U.S. Pat. No. 2,069,060 (Jan. 26, 1937) and U.S. Pat. No. 3,384,454 (May 21, 1968) disclose the preparation of sintered, alumina materials of 0.02-0.05% residual Na.sub.2 O content by incorporating B.sub.2 O.sub.3 in alumina hydrate of 0.1-0.6% Na.sub.2 O content, followed by calcination of the mixture in excess of about 1300.degree. C.
Sintered alumina can be prepared by calcination of alumina hydrate at temperatures in excess of about 1400.degree. C., generally in the range of 1600.degree. and 1900.degree. C. Sintering of the commonly available alumina hydrate having an average particle size in the range of about 25-50 microns, results in a product of high porosity generally in the range of 10-15% by volume and a density which renders it unsuitable for many applications. To improve these properties of the sintered alumina it has been found advantageous in the past to subject the hydrate to an intermediate calcination step, for example, at 900.degree.-1100.degree., followed by a particle size reduction treatment, such as grinding, to an average particle size of about 1-10 microns. The ground, calcined alumina, commonly referred to as refractory grade alumina, is then sintered and the produced product will have slightly improved density and porosity (7-12% by volume) characteristics. The improvements attained by the above-described treatment were still found to be inadequate. To further improve the porosity characteristics of sintered aluminas made from refractory grade alumina the prior art has incorporated densifiers in the ground calcined alumina prior to sintering. The densifiers, generally magnesium compounds, act as grain growth inhibitors during the sintering process and produce a sintered alumina of increased density and reduced porosity. Thus, the prior art has prepared sintered, refractory grade aluminas having a porosity of 5-9% by volume by addition of about 1-5% by weight magnesium salt to the alumina. While the addition of a magnesium-based densifier improved the porosity of the sintered, refractory grade alumina, the incorporation of a relatively high quantity of magnesium cation in the alumina resulted in a product of reduced utility. To overcome the difficulties associated with the prior art processes a method has been suggested in U.S. Pat. No. 3,655,330 (Apr. 11, 1972) resulting in a high purity and high density sintered alumina. According to this reference in order to obtain a sintered alumina of high purity and density, an alpha-alumina precursor, such as high purity Al(NO.sub.3).sub.3. 9H.sub.2 O, is milled in a fluid energy mill, followed by calcination at 800.degree.-1200.degree. C. in an atmosphere of steam. The calcined product is mixed with 0.1% grain growth inhibitor, such as a magnesium salt, then this mixture is shaped at 2000-30,000 p.s.i.g., followed by sintering at about 1300.degree.-1600.degree. C. The resulting product, as shown, exhibits purities in excess of 99.9% and skeletal densities greater than 3.92 g/cc. In order to obtain these desirable properties an extremely pure precursor has to be employed and the precursor has to be calcined in an atmosphere of steam. The requirement of high purity precursor and the required process steps render this process economically unattractive, particularly where large quantities of sintered, high purity and dense alumina are to be produced for use in the refractories industry.
It has now been found that a high purity and dense sintered alumina can be readily made from a Bayer process alumina hydrate precursor of 0.2-1.0% by weight Na.sub.2 O content (on Al.sub.2 O.sub.3 basis) by incorporating in calcined alumina made from the precursor, a synergistically acting combination of magnesium salt and boron compound followed by sintering at about 1650.degree.-1850.degree. C. The synergistic effect of the mixture is exhibited in the extremely low residual Na.sub.2 O level of the sintered alumina which cannot be achieved by the addition of the boron compound alone; the porosity of the sintered product, due to the synergism of the mixture, can also be reduced to levels unattainable by the single addition of a magnesium salt densifier.